1. Field of the Invention
The present invention relates to methods for preparing products by removal of a solid frozen solvent from a frozen matrix mixture.
2. Description of the Background Art
Freeze-drying, i.e., lyophilization, is a well known method of drying heat-sensitive materials in order to protect them from thermal damage. In the past, preparations containing active ingredients, such as pharmaceuticals, nutrients, diagnostics, fertilizers and insecticides, have been prepared by freeze-drying aqueous solutions or suspensions containing these bioactive ingredients. Conventional methods of freeze-drying or lyophilization involve the freezing of a material at a very low temperature followed by a dehydration by sublimation under high vacuum. These conventional techniques generally employ expensive, bulky and specially designed lyophilization equipment.
One problem that has arisen, however, with the use of conventional freeze-drying processes is cracking of the freeze-dried preparations. Typically, cracking is caused by the stresses set up during ice crystallization. Though cracking is never desirable, it is especially undesirable where drop methods of freezing are employed. In such cases, cracking of the frozen droplets usually results in unusable and inelegant remnants of fractured droplets.
Another problem encountered by use of known freeze-drying methods is a phenomenon called meltback. Meltback occurs when the heat required during the drying process melts the frozen material. As such, meltback defeats the whole purpose of freeze-drying--the removal of water through sublimation as opposed to evaporation. To avoid meltback in conventional freeze-drying methods, only limited amounts of material of limited thickness can be dried at one time or, alternatively, very low temperatures have to be used, thereby considerably extending the time required for sublimation. Even with these limitations, conventional freeze-drying methods are not always sufficient to prevent meltback.
Yet another problem inherent in conventional freeze-drying methods is a lack of resistance to disintegration in freeze-dried materials, i.e., they have little strength. Freeze-drying methods generally yield products that merely crumble when handled. Various freeze-drying and packaging methods have been employed in attempts to circumvent this problem. For example, U.S. Pat. No. 4,305,502 describes a method for forming a shaped article by a lyophilization process in a depression in a sheet of film material. However, such packaging techniques do not avoid the problems posed by conventional freeze-drying methods; the tablets are still susceptible to crumbling if transferred to other packaging.
In the area of pharmaceuticals, known freeze-dried dosage forms do not always exhibit fast dissolution rates when brought into contact with appropriate solvents, such as water, saliva or gastrointestinal fluids. Rapid dissolution of pharmaceutical dosage forms can be of critical importance in instances where it is desirable that the pharmaceutical enter the physiological system as soon as possible.
For example, many individuals, particularly pediatric and geriatric patients, experience difficulty and discomfort in swallowing solid, slow dissolving tablets and capsules. Similar difficulties are encountered when administering pharmaceuticals orally to animals in the veterinary treatment of those animals.
Various methods for freeze-drying pharmaceutical dosage forms by lyophilization have been developed to provide fast dissolving dosage forms. U.S. Pat. Nos. 2,166,074; 3,234,091; 4,371,516 and 4,302,502 and United Kingdom Patents No. 698,767 and 1,310,824 are all concerned with freeze-dried dosage forms that are able to dissolve rapidly. In addition, Davies, in U.S. Pat. No. 4,642,093, teaches a procedure for preparing a freeze-dried (lyophilized) foam dosage form using conventional lyophilization techniques that results in rapidly dissolving pharmaceutical dosage forms.
Yet another problem intrinsic to conventional lyophilization methods is the lack of uniform porosity in the lyophilized product. Uniform porosity in a lyophilized product is critical for post-loading a dosage form with an active agent.
Thus, there is a need for a method of producing a dosage form similar to that produced by lyophilization that will avoid cracking and meltback. Furthermore, there is a need for methods of producing pharmaceutical dosage forms having adequate strength, porosity and exhibiting a fast speed of dissolution upon ingestion.